Human lymphocytes and monocytes have been shown to exhibit mixed function oxidase activity, a property which in many tissues and species is responsible for the activation of polycyclic hydrocarbon carcinogens. The levels of this activity vary among individuals and may be enhanced by culturing the cells with various inducers. In rat and rabbit tissues cytochrome P-450 is present in multiple forms and these forms differ in their substrate specificities. Multiple forms of cytochrome P-450 have not yet clearly been demonstrated in human tissues. Antibodies prepared against two forms of rat liver cytochrome P-450 have been found to inhibit mixed function oxidase (i.e., benzo(a)pyrene hydroxylase) activity of human lymphocytes and monocytes. In this study the specificity of this inhibition is being investigated further.